What environmental issue is a consequence of excessive agricultural runoff into coastal waters?

Excessive agricultural runoff into coastal waters primarily leads to nutrient pollution, which causes algal blooms. This process occurs when fertilizers containing high levels of nitrogen and phosphorus are washed into water bodies, often from rain or irrigation. The increase in nutrients fuels the rapid growth of algae.

When these algal blooms occur, they can deplete the oxygen in the water, leading to hypoxic conditions, commonly known as "dead zones." These areas have insufficient oxygen to support aquatic life, which can harm fish and other marine organisms, disrupt ecosystems, and negatively impact local fisheries and wildlife. Moreover, some types of algal blooms can produce toxins that are harmful to marine animals and humans, further exacerbating the environmental issues in coastal areas.

While other options like oil spills, plastic pollution, and thermal pollution are significant environmental concerns, they are not directly linked to agricultural runoff in the same way that nutrient pollution is. Thus, the correct answer highlights a specific ecological consequence tied to agricultural practices and their impact on coastal ecosystems.